The Revolutionary Guards are pretty enormous and pretty fanatical.
The entire country isn't represented by Tehran College Students.
Opposition groups at times have a tendency to exaggerate how easily a government could be overthrown (Bay of Pigs, anyone?)
On the other hand repressive governments can at times fall with shocking ease.
I'm very skeptical of Iran though.
Rule #1
Emigres will tell you whatever you want to hear. They will exaggerate their popularity back home. They will lie like rugs because they want you to do their fighting for them.
I see the potential for a lot of Stalingrad-esque house to house, CQB. Minus tanks (on the Iranian side) after we attempt to take out selected targets with PGM, etc; the whole country will have to be taken brick by brick, nail by nail. All soldiers should have a back up weapon, plenty of extra ammo, virtually a decade's supply of M-67 grenades and a K-Bar always at the ready. I would expect a long haul in Iran with a "forest fire" potentially rekindled in Iraq.
What many analysts may not foresee is that the insurgency in Iraq (and elsewhere) could in fact be fanned if we get bogged down in Iran. Insurgents and foreign fighters may be emboldened to come crawling out of the woodwork to a hitherto unforeseen extent (if we are perceived to be stretched too thin between Iraq and Iran). Hopefully all Iraqi security forces will stay the course.. We could have a modern day battle of the Teutoberg forest on our hands if we are not extremely careful.
Iran is no Romania/Nikolai Ceausescu. Finesse will not work, but brutal, sustained, up close in your face ops might, given enough men and materiel. PSYOPS will always be needed to help prepare the battle space and hopefully fan the internal opposition in Iran, but I am skeptical that they all want free, open democracy in Persia. I could be wrong and sincerely hope that I am if we have to go in.